Smallpox vaccines: Past, present, and future.

The global eradication of smallpox was a tremendous achievement made possible by the development of an effective vaccine. Routine vaccination of the general population is no longer recommended. However, stocks of variola virus, the causative agent of smallpox, still exist in 2 secure laboratories, and permanent disposal has been controversial. In addition, there is speculation that variola virus may exist outside of these 2 facilities, and there is a concern that the threat of smallpox will be used as a bioterrorist weapon. In 2002, this concern led to a vaccination campaign in US military and civilian healthcare workers and first responders. Although the historical live virus vaccine has proven efficacy, it also is associated with serious adverse events and rare fatal reactions, particularly in the setting of immunodeficiency and atopic eczema. In addition, this vaccine was historically produced using animal intermediaries in a process that was prone to contamination and not acceptable for current manufacturing standards. Development of alternative poxvirus vaccines is focused on replication-defective viruses, gene-based vectors, and subunit approaches to improve safety and immunogenicity. The conundrum is that in the absence of an intentional release of variola, efficacy evaluation of new candidate vaccines will be limited to animal model testing, which creates new challenges for the vaccine licensure process. Although motivated by the threat of bioterrorism, the hope is for new poxvirus vaccines to have their greatest utility against other pathogenic orthopoxviruses such as monkeypox and for the development of recombinant poxvirus-based vectors to treat and prevent other diseases.